Blip, Ping, and Buzz: Making Sense of Radar and Sonar by Mark Denny

Author:Mark Denny [Mark Denny]
Language: eng
Format: epub, pdf
Published: 2011-01-15T10:40:00+00:00

Figure 5.9 Spectrogram of two bat echolocation signals, recorded together. The horizontal axis is time, covering an interval of 0.374 s. (a) The amplitude or power of the signals is represented by the width of the trace. (b) CF signal of a horseshoe bat (flattopped pulses)-note the high duty cycle, that is, the large fraction of time spent transmitting. Also shown are the much shorter FM pulses (like inverted commas) that descend in frequency. I thank Prof Brock Fenton for supplying this graph.

In figure 5.9 you can see a spectrogram of a typical transmitted CF pulse. Because of the long duration of CF pulses, and the unvarying frequency within each pulse, these waveforms have a very well defined frequency. So do the corresponding echo signals, though these echoes will in general be Doppler-shifted because of the relative movement of bat and target. CF bat hearing is particularly acute over a very narrow band of frequencies, an acoustical sweet spot, and there is a danger that the ever-changing movements of predator and prey might Doppler shift the echoes out of the sweet spot. But CF bats get around this potential problem: they adjust their transmitter frequency on the fly (literally) so that the echoes always hit the sweet spot, no matter what the bat-prey relative speed might be. Such dynamical adjustment is a difficult trick, but it pays great dividends, because the sweet spot is able to resolve very small frequency differences in the echo signal, and the bat uses this information to classify the target.

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